1. Field of the Invention
This invention relates to a plastic spherical lens having a multiplicity of focal points, and a method of its production. More specifically, it relates to a multifocal point spherical lens comprising a resin layer having a refractive index gradient nearly throughout the lens, and a small lens portion formed of a resin having a different refractive index from a resin forming the matrix of the spherical lens, and to a method of its production.
2. Description of the Prior Art
With a rapid increase in the proportion of aged people in the total population in recent years, there has been an increasing demand for eye glasses having bifocal, multifocal or cumulative multifocal points.
In the prior art, bifocal or multifocal point eyeglass lenses are generally produced by forming on an eyeglass lens having a fixed radius of curvature a small lens portion having a different radius of curvature by complex machining or by using a special eyeglass lens mold.
However, conventional general bifocal or multifocal point lenses are not suitable for use as eyeglass lenses which also require fashionability as an important element because the protrusion of the small lens portion is conspicuous on the surface of the parent lens.
Furthermore, these lenses including cumulative multifocal point lenses require complex machining, and the cost of production becomes very high Hence, the increased price of the final products is a major cause of retarded acceptance of such lenses despite a large latent demand for them.
Japanese Laid-Open Patent Publication No. 201216/1982 describes a multifocal point organic lens having a non-uniform refractive index consisting of a multifocal point organic lens substrate having a uniform refractive index and an organic substance having a different refractive index from the lens substrate diffused and polymerized on either one of a far view portion or a near view portion of the lens substrate. According to the description of this patent document, this multifocal point organic lens is produced by masking the surface of the parent lens except a region on which to form the small lens, applying an organic substance having a different refractive lens from the parent lens to the small lens-forming region, and diffusing the organic substance in the parent lens and polymerizing it to thereby form a small lens portion having a refractive index gradient. It is very difficult, or virtually impossible, however to diffuse the organic substance only in the unmasked portion and polymerize it. After the end of polymerization, an after-step of polishing and removing the resulting polymer of the organic substance formed in unwanted parts is required. Furthermore, this method can produce only a spherical lens with a relatively small area of the small lens portion.
Japanese Laid-Open Patent Publication No. 120217/1983 proposes a method of producing a bifocal or multifocal point lens having a fixed radius of curvature which comprises producing a parent lens so that it can receive a small lens therein, fixing the small lens to the parent lens by bonding or otherwise, and then polishing the entire parent lens to a uniform curvature. According to this method, the adhesion between the parent lens and the small lens is a problem, and this problem is particularly difficult to solve when the materials of the parent lens and the small lens have a large difference in the coefficient of thermal expansion. Furthermore, since the thickness of the parent lens cannot be made too large in view of fashionability or weight, it is impossible to increase the thickness of the small lens portion or to reduce the radius of curvature of the small lens portion too much. For this reason, it is difficult to change the focal length of the small lens portion greatly from that of the parent lens by the above method, and further, this method essentially requires an after-step of polishing the entire lens surface. Thus, when it is desired to obtain a lens having greatly varying focal lengths, it is necessary to select a monomer (or a monomeric mixture) which when converted to a polymer, has a refractive index greatly different from that of the parent lens, and also possesses other properties which make it withstand use as an eyeglass In the case of ordinary bifocal, multifocal, or cumulative focal point eyeglass lenses, the focal length of the small lens should be made shorter than that of the parent lens Hence, the above method requires selection of a monomer (or a monomeric mixture) having a very high refractive index Since in this case, dispersion becomes considerably great, the resulting lens has the defect of marked chromatic aberration
Recently, cumulative multifocal point eyeglass lenses have attracted attention which are produced by working eyeglass lenses having a fixed radius of curvature to render them non-spherical, or by using an eyeglass lens mold having a special non-spherical shape. Such methods have the same problems as described above.